The present disclosure relates to a modular multi-level converter, and more particularly, to a method of controlling voltage balancing.
Since high voltage direct current (HVDC) transmission has advantages as compared with high voltage alternating current transmission (HVAC) in that it is possible to perform long-distance transmission and an asynchronous grid connection, use a submarine cable, and perform power control, its applications are steadily increasing.
In an HVDC transmission system, at a transmission side converts and transmits AC power into DC power, and a reception side converts the DC power into the AC power to supply the AC power to a consumer.
Thus, the HVDC transmission system essentially includes a converter in order to convert the AC power into the DC power or convert the DC power into the AC power.
Such a converter includes six arms D1 to D6 as shown in FIG. 1. AC power is converted into DC power by the switching control of respective arms D1 to D6.
Each of arms D1 to D5 includes a switch. However, there is a limit in a voltage that a single switch may stand.
Thus, a modular multi-level converter has been recently proposed in which each of arms D1 to D6 includes a plurality sub modules and which may thus stand a high voltage by the selective switching control of each sub module.
Each sub module includes two insulating gate bipolar transistor (IGBT) and a capacitor.
The number of sub modules that each of arms D1 to D6 includes may be determined according to the processing capacity of the modular multi-level converter and may be up to several hundreds. However, it is not easy to perform selective switching control on the sub module.
In particular, the voltage value of the capacitor in the sub module is not fixed due to an operation condition but varies. In addition, in the case where a large number of sub modules are manufactured by different manufacturers, the specification of the capacitor of each sub module may be different from each other. Thus, the capacitance of the capacitor of each sub module may be different from each other and due to such a difference in the capacitance of the capacitor, the capacitance voltage of the capacitor of each sub module may vary. The capacitor value is a voltage charged in the capacitor.
Since the charging time of the voltage charged in the capacitance is different from each other when the capacity of the capacitance of each sub module is different from each other, the switching frequency of a sub module that has low capacitance sharply increases, and the switching frequency of a sub module that has high capacitance sharply decreases. If the switching frequency of each sub module sharply increases or decreases in this way, there is a drawback in that the life of the IGBT in the sub module shortens.
Since it makes the life of the sub module short, there is an urgent need for a technology that constantly maintains the capacitor voltage of the sub module.